When the form is submitted, your server will receive two extra form fields:
captcheck_session_code and captcheck_selected_answer.
In your form handling code, send a request to http(s)://captcheck-url/api.php.
Pass the variables session_id and answer_id with the values sent with the form,
and also pass the variable action with the value verify.
You will receive a JSON response with (among other things) "result": true or
"result": false. If result is false, the user failed the test, and another
variable msg is available with an explanation.